The present invention relates generally as a semiconductor integrated circuit constructed to a single chip, and more particularly, to a semiconductor integrated circuit constructed as a gate array or the like for use for various specific applications.
JP-A-72154/1987 describes the prior art associated with the present invention. The above document describes that
(a) a control portion in a conventional laser beam printer could not be made small in size or weight because control circuits for an interface control portion and a mechanism control portion were respectively constituted by approximately 40 general-purpose discrete ICs, and proposes
(b) techniques for making the laser beam printer small in size and weight by forming each of the control circuits, which were respectively made as general-purpose discrete ICs, into a one-chip semiconductor integrated circuit.
The adoption of this proposal allows the control portion in the laser beam printer to be reliably miniaturized. However, the prior art has a disadvantage in that the operating speed of logic circuits formed into a one-chip semiconductor integrated circuit is determined by characteristics of semiconductor materials and devices (generically referred to as "component devices" hereinafter) forming the semiconductor integrated circuit. More specifically, the maximum operating frequency inherent in the component devices is determined by the characteristics thereof. Accordingly, the operating frequency of the control circuit formed as a one-chip semiconductor integrated circuit is restricted by the maximum operating frequency of the characteristics of the component devices.
Meanwhile, in a laser beam printer or the like, studies for improving the resolution of these shortcomings have been conducted day after day. Consequently, printers superior in resolution have been commercialized. For example, at the present time, a laser beam printer capable of outputting 12 to 16 lines per 1 mm is commercially available. In such a laser beam printer, the control portion must output a control signal for drawing 12 to 16 times per 1 mm. Thus, high-speed operation of the control portion is required as the resolution of the laser beam printer is improved. When a high-speed operation of this control portion is achieved, it is necessary to count signals having a high frequency to define a horizontal position where dots are to be drawn, for example, in a control circuit for controlling operation of a laser diode.
Furthermore, if the number of sheets of paper to be printed per unit time by the laser beam printer is increased, the operating speed of each of control circuits constituting the control portion must be increased.
If a lot of general-purpose discrete ICs are formed into a one-chip semiconductor integrated circuit by adopting the above described proposal (b), however, there occurs the following problem: If a higher operating frequency than the maximum operating frequency determined by the characteristics of the component devices forming the semiconductor integrated circuit is required by the improvement in resolution of the printer, a semiconductor integrated circuit capable of dealing with a higher-speed operation must be newly produced by another semiconductor process. More specifically, in a case where an integrated circuit is formed of component devices having the maximum operating frequency required in the present stage, and the resolution of the printer is to be improved in the near future, a higher operating frequency than the maximum operating frequency of the component devices is required, and a semiconductor integrated circuit must be newly produced again.
On the other hand, it has been suggested to construct an integrated circuit using component devices having a maximum operating frequency higher than the operating frequency required in the present stage in view of the above described possible improvement in resolution of the printer. However, that is not desirable, because it costs a lot for matters which are not required in the present stage and may not necessarily be required in the near future, thereby leading to an increased cost for the semiconductor integrated circuit.
Similar technical subject is common to a semiconductor integrated circuit for a laser beam printer and various semiconductor integrated circuits for specific applications; that is, semiconductor integrated circuit referred to as ASICs (Application Specific Integrated Circuits).